A sintered R-T-B based magnet, including an R2T14B type compound as its main phase, is known as a permanent magnet with the highest performance, and has been used in various types of motors such as a motor for a hybrid car and in numerous types of consumer electronic appliances.
As a sintered R-T-B based magnet loses its coercivity at high temperatures, such a magnet will cause an irreversible flux loss. For that reason, when used in a motor, for example, the magnet should maintain coercivity that is high enough even at elevated temperatures to minimize the irreversible flux loss.
It is known that if R in the R2T14B type compound is replaced with a heavy rare-earth element RH, the coercivity of a sintered R-T-B based magnet will increase. It is effective to add a lot of such a heavy rare-earth element RH to the sintered R-T-B based magnet to achieve high coercivity at a high temperature.
However, if the light rare-earth element RL is replaced with the heavy rare-earth element RH as R in a sintered R-T-B based magnet, the coercivity (which will be referred to herein as HcJ) certainly increases but the remanence (which will be referred to herein as Br) decreases instead. Furthermore, as the heavy rare-earth element RH is one of rare natural resources, its use should be cut down.
For these reasons, various methods for increasing HcJ of a sintered R-T-B based magnet effectively with the addition of as small an amount of the heavy rare-earth element RH as possible without decreasing Br have recently been researched and developed.
Patent Document No. 1 discloses a method for producing a sintered R-T-B based magnet which is designed to diffuse a heavy rare-earth element RH such as Dy or Tb inward from the surface of a magnet material and increase HcJ without decreasing Br by performing the steps of: loading the sintered R-T-B based magnet body and an RH diffusion source which is a metal or alloy of a heavy rare-earth element RH into a processing chamber so that the sintered R-T-B based magnet body and the RH diffusion source are movable relative to each other and brought close to, or in contact with, each other, and heating the sintered R-T-B based magnet body and the RH diffusion source to a temperature of 500° C. to 850° C. for 10 minutes or more while moving the sintered R-T-B based magnet body and the RH diffusion source in the processing chamber either continuously or discontinuously.
On the other hand, Patent Document No. 2 discloses a method for producing a rare-earth magnet with increased HcJ by performing a first step of depositing a heavy rare-earth compound including an iron compound of Dy or Tb on a sintered rare-earth magnet body and a second step of thermally treating the sintered rare-earth magnet body on which the heavy rare-earth compound has been deposited.